Aluminum-titanate-based ceramic honeycomb structure, its production method, and starting material powder for producing same

ABSTRACT

A ceramic honeycomb structure having a large number of flow paths partitioned by porous cell walls, the cell walls comprising at least main crystals of aluminum titanate, in which MgO and SiO 2  are dissolved to form a solid solution, and glass phases, the glass phases containing 40-80% by mass of SiO 2  and 1-20% by mass of MgO, the average size of the glass phases being 30 μm or less in a cross section of the cell walls, and the area ratio of the glass phases to the total area of the main crystals of aluminum titanate and the glass phases being 2-12% in a cross section of the cell walls, and its production method.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/JP2008/070821 filed Nov. 14, 2008, claiming priority based onJapanese Patent Application No. 2007-295914 filed Nov. 14, 2007 andJapanese Patent Application No. 2008-060465 filed Mar. 11, 2008, thecontents of all of which are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

The present invention relates to an aluminum-titanate-based ceramichoneycomb structure, its production method, and a starting materialpowder for producing it, particularly to an aluminum-titanate-basedceramic honeycomb structure suitable for a ceramic honeycomb filter forcleaning PM-containing exhaust gases discharged from diesel engines,etc., its production method, and a starting material powder forproducing it.

BACKGROUND OF THE INVENTION

An exhaust gas emitted from diesel engines contains PM (particulatematter) based on carbonaceous soot and SOF (soluble organic fraction) ofhigh-boiling-point hydrocarbons. When such exhaust gas is released intothe atmosphere, it may adversely affect human beings and theenvironment. For this reason, a PM-capturing ceramic honeycomb filter,which may be called “honeycomb filter” in short, has been disposed in anexhaust pipe connected to a diesel engine. One example of honeycombfilters for purifying an exhaust gas by removing PM is shown in FIGS. 1(a) and 1(b). The honeycomb filter 10 comprises a ceramic honeycombstructure (simply called honeycomb structure) comprising porous cellwalls 2 defining large numbers of outlet-side-sealed flow paths 3 andinlet-side-sealed flow paths 4, and an outer peripheral wall 1, andupstream-side plugs 6 a and downstream-side plugs 6 c alternatelysealing the outlet-side-sealed flow paths 3 and the inlet-side-sealedflow paths 4 on the exhaust-gas-inlet-side end 8 and theexhaust-gas-outlet-side end 9 in a checkerboard pattern.

Honeycomb filters are required to capture particulate matter fromexhaust gases at high efficiency, with low pressure loss to reduceburden on engines. However, the more particulate matter captured, thehigher pressure loss occurs due to the clogging of cell wall pores.Accordingly, it is necessary to burn off the captured particulate matterto regenerate honeycomb filters. Because the honeycomb filters are thusrepeatedly exposed to high temperatures to burn particulate matter, theyare required to have high heat resistance and high heat shockresistance. Materials for forming porous cell walls have conventionallybeen cordierite (5SiO₂-2Al₂O₃-2MgO) and silicon carbide (SiC).

Cordierite cell walls are resistant to heat shock cracking, meaningexcellent heat shock resistance, because of as low a thermal expansioncoefficient as about 10×10⁻⁷/° C., but the combustion of muchaccumulated particulate matter puts honeycomb filters at too hightemperatures, making it likely that the cell walls are partially melted.Accordingly, precise control is needed to avoid excess elevation of theburning temperatures, resulting in increase in the production andrunning costs of exhaust-gas-cleaning apparatuses.

When cell walls are formed by silicon carbide, honeycomb structures maybe cracked by heat shock stress despite excellent heat resistance,because of as large a thermal expansion coefficient as 40×10⁻⁷/° C. Toreduce heat stress, divided honeycomb filter parts may be integrallybonded, but its effects are not sufficient, only suffering a high costdue to division and bonding.

Recently proposed to solve the above problems is the use of aluminumtitanate (Al₂TiO₅) for honeycomb structures. Aluminum titanate has heatresistance exceeding 1700° C., a small thermal expansion coefficient,and excellent heat shock resistance. Aluminum titanate has a smallthermal expansion coefficient because of microcracks generated in asintering process due to the anisotropic thermal expansion coefficientof aluminum titanate crystals, while the microcracks reduce themechanical strength of the honeycomb structures. Namely, conventionalaluminum titanate suffers contradiction to meet a small thermalexpansion coefficient and high strength. Also, because conventionalaluminum titanate is usually decomposed in a temperature range of800-1280° C., it cannot be used stably in this temperature range for along period of time. To solve such problems of conventional aluminumtitanate, the following technologies are disclosed.

WO 05/105704 discloses an aluminum magnesium titanate crystal structurehaving a thermal expansion coefficient of −6×10⁻⁶ to 6×10⁻⁶ (1/K), whichis formed by a solid solution, in which part of Al atoms on at least asurface layer of an aluminum magnesium titanate crystal having acomposition represented by Mg_(x)Al_(2(l-x))Ti_((1+x))O₅, wherein0.1≦x<1, are substituted by Si atoms, the ratio of aluminum magnesiumtitanate remaining when kept at 1100° C. for 300 hours in the air being50% or more, and its production method. WO 05/105704 describes that theabove structure has heat resistance inherent in aluminum titanate and anextremely small thermal expansion coefficient, as well as excellent heatshock resistance, high thermal decomposability and high mechanicalstrength. However, higher-performance honeycomb filters are recentlydemanded, and the honeycomb filter described in WO 05/105704 isinsufficient to meet both a low thermal expansion coefficient and highstrength, and not satisfactory in pressure loss characteristics.Accordingly, the method of WO 05/105704 cannot produce a honeycombfilter having heat shock resistance, strength, high-temperaturestability and pressure loss characteristics improved to practicallyacceptable levels.

WO 06/39255 discloses a ceramic body comprising 50-95% by mass ofaluminum titanate crystal phases and 5-50% by mass of glass phases, theglass phases having a composition comprising 50-90% of SiO₂, 1-25% ofAl₂O₃, 0.5-10% of TiO₂, 0.5-20% of R₂O, wherein R is an element selectedfrom the group consisting of Li, Na, K, Ru, Cs and Fr, and 0.5-20% ofR′O, wherein R′ is an element selected from the group consisting of Be,Mg, Ca, Ba and Ra, and its production method, and describes that theceramic body has excellent shock resistance and heat cycle resistanceand is suitably usable at high temperatures. However, recent demand ofhigher-performance honeycomb filters makes the honeycomb of WO 06/39255insufficient to meet both a low thermal expansion coefficient and highstrength, and unsatisfactory in pressure loss characteristics andthermal stability at 800-1250° C. Accordingly, the method of WO 06/39255cannot produce honeycomb filters having heat shock resistance, strength,high-temperature stability and pressure loss characteristics improved topractically acceptable levels. Though the production method is notdescribed in detail, Examples indicate that pulverized glass of aparticular composition, which is fused at 1600° C., should be used as astarting material, suffering a high production cost.

JP 5-85818 A discloses aluminum titanate ceramics having crystal phasescomposed of 60-85% of aluminum titanate, 10-25% of rutile, 2-10% ofcorundum and 2-10% of mullite, and 5% or less of glass phases, and itsproduction method, and describes that the aluminum titanate hasexcellent heat cycle durability and insert-castability. However, becausethe aluminum titanate ceramics of JP 5-85818 A contain as much rutile as10-20%, they have large thermal expansion coefficients and poor heatshock resistance. In addition, in view of recent demand ofhigher-performance honeycomb filters, its pressure loss characteristicsand thermal stability at 800-1250° C. are not satisfactory. Accordingly,the method of JP 5-85818 A cannot produce honeycomb filters having heatshock resistance, strength, high-temperature stability and pressure losscharacteristics improved to practically acceptable levels.

JP 60-5544 B discloses a silicate-containing aluminum titanate ceramicmaterial having a chemical composition comprising 50-60% by weight ofAl₂O₃, 40-45% by weight of TiO₂, 2-5% by weight of kaolin and 0.1-1% byweight of magnesium silicate, and made of starting materials havingparticle sizes of 0.6 μm or less, and describes that it has high shockresistance and mechanical strength. However, recent demand ofhigher-performance honeycomb filters makes the honeycomb of JP 60-5544 Binsufficient to meet both a low thermal expansion coefficient and highstrength, and unsatisfactory in pressure loss characteristics andthermal stability at 800-1250° C. Accordingly, the method of JP 60-5544B cannot produce honeycomb filters having heat shock resistance,strength, high-temperature stability and pressure loss characteristicsimproved to practically acceptable levels.

JP 2006-96634 A discloses a porous aluminum titanate ceramic body havingporosity of 51-75%, an average pore size of 10-40 μm, and a prescribedpore size distribution, and describes that such structure can provideceramic honeycomb filters with excellent heat resistance and heat shockresistance, low pressure loss and practically acceptable strength.However, recent demand of higher-performance honeycomb filters makes theporous ceramic body of JP 2006-96634 A unsatisfactory in thermalstability at 800-1250° C. It is also insufficient to meet both a lowthermal expansion coefficient and high strength, and further improvementis needed to obtain honeycomb filters having heat shock resistance,strength, high-temperature stability and pressure loss characteristicsimproved to practically acceptable levels.

As described above, conventional aluminum titanate is insufficient tomeet both a low thermal expansion coefficient and high strength, andfurther improvement is needed to obtain honeycomb filters having heatshock resistance, strength, high-temperature stability and pressure losscharacteristics improved to practically acceptable levels.

OBJECT OF THE INVENTION

Accordingly, an object of the present invention is to provide analuminum-titanate-based ceramic honeycomb structure having excellentheat shock resistance, pressure loss characteristics and thermalstability at 800-1250° C. and improved strength, its production method,and a starting material powder for producing it.

DISCLOSURE OF THE INVENTION

As a result of intensive research in view of the above object, theinventors have found that by regulating the composition, size and ratioof glass phases in a ceramic honeycomb structure comprising maincrystals of aluminum titanate and glass phases, large numbers of finemicrocracks are generated in a sintering process, thereby providing thehoneycomb structure with a small thermal expansion coefficient andimproved strength. The present invention has been completed based onsuch finding.

Thus, the aluminum-titanate-based ceramic honeycomb structure of thepresent invention has a large number of flow paths partitioned by porouscell walls, the cell walls comprising at least main crystals of aluminumtitanate, in which MgO and SiO₂ are dissolved to form a solid solution,and glass phases; the glass phases containing 40-80% by mass of SiO₂ and1-20% by mass of MgO; the average size of the glass phases being 30 μmor less in a cross section of the cell walls; and the area ratio of theglass phases to the total area of the main crystals of aluminum titanateand the glass phases being 2-12% in a cross section of the cell walls.

Another aluminum-titanate-based ceramic honeycomb structure of thepresent invention has a large number of flow paths partitioned by porouscell walls, the cell walls comprising at least main crystals of aluminumtitanate, in which MgO and SiO₂ are dissolved to form a solid solution,and glass phases; the glass phases containing 40-80% by mass of SiO₂ and1-20% by mass of MgO; the average size of the glass phases being 30 μmor less in a cross section of the cell walls; and the mass ratio of theglass phases to the total of the main crystals of aluminum titanate andthe glass phases being 1% or more by mass and less than 10% by mass.

The amounts of MgO and SiO₂ dissolved in the main crystals arepreferably 0.2-5% by mass and 0.1-1.5% by mass, respectively.

The cell walls preferably have porosity of 40-70%, an average pore sizeof 10-40 μm, and a pore distribution deviation σ[=log(D20)-log(D80)] of0.4 or less, wherein D20 represents a pore size (μm) at a pore volumecorresponding to 20% of the total pore volume, and D80 represents a poresize (μm) at a pore volume corresponding to 80% of the total porevolume, both in a curve representing the relation between the pore sizeand the cumulative pore volume (pore volume accumulated in a range fromthe maximum pore size to a particular pore size), and D80<D20.

The cell walls preferably have air permeability of 2×10⁻¹² m² or more.

The honeycomb structure preferably has an A-axis compression strength of4 MPa or more. The cell walls preferably have a thermal expansioncoefficient of 12×10⁻⁷/° C. or less. When the cell walls are kept in anatmosphere at 1100° C.100 hours, the ratio of the remaining aluminumtitanate is preferably 95% or more.

The method of the present invention for producing analuminum-titanate-based ceramic honeycomb structure comprises the stepsof mixing 100 parts by mass of a powder material comprising aluminapowder containing 0.05-0.5% by mass of Na₂O and titania powdercontaining 0.5% or less by mass of Na₂O at a molar ratio of 47/53 to53/47, with at least 1-6 parts by mass of silica powder and 0.5-5 partsby mass of magnesia source powder having an average particle size of 5μm or less; molding the mixed powder material to a honeycomb structure;heating the resultant molding at an average speed of 10° C./hr to 100°C./hr in a temperature range of 1300° C. to 1350° C.; and sintering itat the highest keeping temperature of 1400-1650° C. to obtain analuminum-titanate-based ceramic honeycomb structure comprising at leastmain crystals of aluminum titanate, in which MgO and SiO₂ are dissolvedto form a solid solution, and glass phases.

The alumina powder preferably has an average particle size of 0.1-10 μm,the titania powder preferably has an average particle size of 0.05-3 μm,and the silica powder preferably has an average particle size of 0.1-30μm. The alumina powder more preferably has an average particle size of2-6 μm.

A pore-forming material preferably is added to the mixed powdermaterial.

The molding is preferably kept at the highest keeping temperature for 24hours or longer.

The starting material powder of the present invention for producing analuminum-titanate-based ceramic honeycomb structure comprising at leastmain crystals of aluminum titanate, in which MgO and SiO₂ are dissolvedto form a solid solution, and glass phases, comprises 100 parts by massof a powder material comprising alumina powder containing 0.05-0.5% bymass of Na₂O and titania powder containing 0.5% or less by mass of Na₂Oat a molar ratio of 47/53 to 53/47, and at least 1-6 parts by mass ofsilica powder, and 0.5-5 parts by mass of magnesia source powder havingan average particle size of 5 μm or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a schematic cross-sectional view showing one example ofhoneycomb filters perpendicularly to flow paths.

FIG. 1( b) is a schematic cross-sectional view showing one example ofhoneycomb filters in parallel to flow paths.

FIG. 2( a) is a SEM photograph showing a cell wall cross section of thehoneycomb structure.

FIG. 2( b) is a schematic view showing a cell wall cross section of thehoneycomb structure with glass phases hatched.

FIG. 2( c) is a view for explaining the method of counting the number ofmicrocracks generated in the crystal phases in the honeycomb structure.

FIG. 3 is a graph showing the relation between pore size and pore volumein the cell walls of the honeycomb structure produced in Example 3.

FIG. 4 is a schematic cross-sectional view showing another example ofhoneycomb filters in parallel to flow paths.

DESCRIPTION OF THE PREFERRED EMBODIMENTS [1] Aluminum-Titanate-BasedCeramic Honeycomb Structure

(1) First Honeycomb Structure

The first aluminum-titanate-based ceramic honeycomb structure of thepresent invention (simply called “honeycomb structure” below) having alarge number of flow paths partitioned by porous cell walls ischaracterized in that the cell walls comprise at least main crystals ofaluminum titanate, in which MgO and SiO₂ are dissolved to form a solidsolution, and glass phases, that the glass phases contain 40-80% by massof SiO₂ and 1-20% by mass of MgO, that the average size of the glassphases is 30 μm or less in a cross section of the cell walls, and thatthe area ratio of the glass phases to the total area of the maincrystals of aluminum titanate, in which MgO and SiO₂ are dissolved toform a solid solution, and the glass phases is 2-12% in a cross sectionof the cell walls.

With MgO and SiO₂ dissolved in main crystals of aluminum titanate, thehoneycomb structure has improved thermal stability at 800-1250° C. With40-80% by mass of SiO₂ and 1-20% by mass of MgO contained in the glassphases, the crystals of aluminum titanate are bonded with the glassphases, and fine microcracks are introduced into cell walls in asintering process, thereby providing the honeycomb structure with asmaller thermal expansion coefficient and improved strength.

The glass phases contain 40-80% by mass of SiO₂ and 1-20% by mass ofMgO. The glass phases can be made amorphous by 40-80% by mass of SiO₂,and provided with low melting points by 1-20% by mass of MgO.Accordingly, a liquid phase is generated in a temperature range in whichaluminum titanate is synthesized from alumina material powder andtitania material powder in a sintering process, resulting in a sinteredcomposite structure having main crystals of aluminum titanate containingMgO and SiO₂ and bonded with fine glass phases. The strong bonding ofthe main crystals of aluminum titanate with fine glass phases makesfiner microcracks generated by the anisotropic thermal expansion ofaluminum titanate, and increases their number. When the amount of MgOcontained in glass phases is less than 1% by mass, the glass phases donot have a sufficiently low melting point, so that the glass phases aremade relatively bulky. When the amount of MgO is 1% or more by mass, themore the amount of MgO, the finer the glass phases. Thus, the dispersionof finer glass phases in the cell walls makes finer microcracksgenerated in the aluminum titanate crystal phases, thereby providing thehoneycomb structure with a smaller thermal expansion coefficient andimproved strength. On the other hand, when the amount of MgO containedin glass phases is more than 20% by mass, the glass phases have too lowa melting point, so that the honeycomb structure has a low softeningtemperature and lose heat resistance inherent in aluminum-titanate-basedceramics.

The average size of the glass phases is 30 μm or less in a cross sectionof cell walls. When the average size is more than 30 μm, the maincrystals of aluminum titanate are not strongly constrained, resulting inlarger microcracks with a smaller number, so that the honeycombstructure does not have a sufficiently low thermal expansion coefficientand sufficient strength. A SEM photograph of a cross section of cellwalls of the honeycomb structure is shown in FIG. 2( a). The glassphases 2 exist in gaps between crystal phases 1 of aluminum titanate asshown by hatching in FIG. 2( b), and their maximum lengths are regardedas the sizes of the glass phases 2. The average size of the glass phases2 are determined by averaging the sizes of glass phases 2 measured at 20sites. The average size of the glass phases is preferably 20 μm or less,more preferably 10 μm or less, most preferably 5 μm or less.

The area ratio of the glass phases to the total area of main crystals ofaluminum titanate, in which MgO and SiO₂ are dissolved to form a solidsolution, and glass phases is 2-12% in a cross section of cell walls.When the area ratio of the glass phases is less than 2%, aluminumtitanate crystals are not strongly bonded, and microcracks are not madesufficiently fine, failing to provide the honeycomb structure withimproved strength. The area ratio of the glass phases exceeding 12%makes the glass phases too influential, providing the honeycombstructure with low strength and a large thermal expansion coefficient.The area ratio of the glass phases is preferably 2-8%, more preferably3-6%.

The glass phases preferably contain, in addition to 40-80% by mass ofSiO₂ and 1-20% by mass of MgO, 10-40% by mass of Al₂O₃, 0.5-10% by massof TiO₂, 0.5-5% by mass of Na₂O, and 0.5-5% by mass of K₂O, Suchcomposition provides the glass phases with a low melting point, andmakes microcracks finer, providing the honeycomb structure with a lowthermal expansion coefficient and high strength. The inclusion of Na₂Oand K₂O both in 0.5-5% by mass makes the melting point of glass phasesfurther lower, so that the honeycomb structure has both low thermalexpansion coefficient and high strength. The glass phases morepreferably contain 50-70% by mass of SiO₂, 2.5-10% by mass of MgO,15-30% by mass of Al₂O₃, 1-8% by mass of TiO₂, 1-4% by mass of Na₂O, and1-4% by mass of K₂O.

0.2-5% by mass of MgO and 0.1-1.5% by mass of SiO₂ are preferablydissolved in the main crystals of aluminum titanate. The dissolving ofMgO and SiO₂ in these ranges improves the thermal stability ofaluminum-titanate-based ceramics at 800-1250° C., and provides aluminumtitanate crystals with improved strength, thereby improving the strengthof the honeycomb structure. The more preferred amount of MgO dissolvedis 0.5-3% by mass, and the more preferred amount of SiO₂ dissolved is0.1-0.6% by mass.

As shown in FIG. 2( c), the number of microcracks 3 can be determined ona SEM photograph showing a cross section of cell walls by aline-crossing method, in which the number of microcracks 3 crossing anarbitrary straight line is counted, and converted to a number per a unitlength of the line excluding that crossing pores. The number ofmicrocracks determined by the line-crossing method is preferably 50 ormore per mm, more preferably 100 or more per mm.

The honeycomb structure substantially comprises main crystals ofaluminum titanate, in which MgO and SiO₂ are dissolved to form a solidsolution, and glass phases containing at least MgO and SiO₂, and it mayfurther contain trace amounts of crystals such as TiO₂, Al₂O₃, spinel,mullite, etc. in ranges not affecting the effects of the presentinvention. Although their amounts are not restrictive, they are 10% orless, preferably 5% or less by mass, based on the total of main crystalsand other crystals. The total amount of crystal phases is determinedfrom an X-ray diffraction pattern of a powdery sample, by the (230)plane diffraction intensity (I_(AT(230))) of aluminum titanate, the(104) plane diffraction intensity (I_(corundum(104))) of Al₂O₃ corundum,the (110) plane diffraction intensity (I_(rutile(110))) of rutile TiO₂,and the (311) plane diffraction intensity (I_(spinel(311))) of spinel,etc. CaO, BaO, Fe₂O₃, Li₂O, SrO, Y₂O₃, etc. may also be contained inranges not affecting the effects of the present invention.

(2) Second Honeycomb Structure

The second aluminum-titanate-based ceramic honeycomb structure of thepresent invention has a large number of flow paths partitioned by porouscell walls, the cell walls comprising at least main crystals of aluminumtitanate, in which MgO and SiO₂ are dissolved to form a solid solution,and glass phases, the glass phases containing 40-80% by mass of SiO₂ and1-20% by mass of MgO, the average size of the glass phases being 30 μmor less in a cross section of the cell walls, and the mass ratio of theglass phases to the total of the main crystals of aluminum titanate andthe glass phases being 1% or more by mass and less than 10% by mass.

With respect to the requirements that the cell walls comprise at leastmain crystals of aluminum titanate, in which MgO and SiO₂ are dissolvedto form a solid solution, and glass phases, that the glass phasescontain 40-80% by mass of SiO₂ and 1-20% by mass of MgO, and that theaverage size of the glass phases is 30 μm or less in a cross section ofthe cell walls, the second aluminum-titanate-based ceramic honeycombstructure is the same as the first one. Accordingly, their explanationwill be omitted, and only the mass ratio of the glass phases will beexplained below.

The mass ratio of the glass phases to the total of main crystals ofaluminum titanate, in which MgO and SiO₂ are dissolved to form a solidsolution, and glass phases is 1% or more by mass and less than 10% bymass. When the mass ratio of the glass phases is less than 1% by mass,aluminum titanate crystals are not bonded strongly, making microcracksinsufficiently fine, and thus failing to improve the strength of thehoneycomb structure. When the mass ratio of glass phases is 10% or moreby mass, the glass phases have large influence, providing the honeycombstructure with small strength and a large thermal expansion coefficient.The mass ratio of the glass phases is preferably 1% or more by mass andless than 5% by mass.

(3) Porous Structure

As described in the section of prior art, it is difficult to provideconventional aluminum titanate with a low thermal expansion coefficientand high strength. Accordingly, it is difficult to obtain alow-pressure-loss honeycomb structure by adjusting porosity, poredistribution, etc., because of insufficient strength. On the other hand,the honeycomb structure of the present invention has fine microcracksbecause of particular glass phases, thereby having high strength whilekeeping low thermal expansion inherent in aluminum titanate ceramics,and further has better pressure loss characteristics than those ofconventional ones by optimizing the porosity or pore distribution ofcell walls.

The aluminum-titanate-based honeycomb structure of the present inventionpreferably has porosity of 40-70%, an average pore size of 10-40 μm, anda pore distribution deviation σ of 0.4 or less. With such a structure,the honeycomb structure is provided with low pressure loss. Here,σ=log(D20)−log(D80), wherein D20 represents a pore size (μm) at a porevolume corresponding to 20% of the total pore volume, and D80 representsa pore size (μm) at a pore volume corresponding to 80% of the total porevolume, both in a curve representing the relation between the pore sizeand the cumulative pore volume (pore volume accumulated in a range fromthe maximum pore size to a particular pore size), as shown in FIG. 3.D80<D20. The relation between the pore size and the cumulative porevolume can be determined by mercury porosimetry.

When the porosity is less than 40%, sufficiently low pressure loss maynot be achieved. When the porosity exceeds 70%, practically sufficientstrength may not be obtained. The more preferred porosity is 45-64%.

When the average pore size is less than 10 μm, sufficiently low pressureloss may not be achieved. When the average pore size exceeds 40 μm, theparticulate-matter-capturing capability may be lowered. The morepreferred average pore size is 15-30 μm.

The aluminum-titanate-based honeycomb structure of the present inventionpreferably has air permeability of 2×10⁻¹² m² or more. The airpermeability is an index indicating how easily air can pass throughporous cell walls. The air permeability of 2×10⁻¹² m² or more providesthe honeycomb structure with low pressure loss.

The pore distribution deviation σ is the difference between thelogarithm of D20 and the logarithm of D80 as described above, indicatingthe pore size distribution. Smaller 6 means a sharper pore sizedistribution. With the porosity of 40-70%, the average pore size of10-40 μm and σ of 0.4 or less, there are many pores contributing to lowpressure loss, and there are few micro-pores deteriorating pressure lossand few coarse pores lowering the strength, thereby providing thehoneycomb structure with low pressure loss and high strength. When thepore distribution deviation σ is more than 0.4, there are a higherpercentage of pores deteriorating pressure loss and strength, failing toobtain a honeycomb structure with low pressure loss and high strength.The pore distribution deviation σ is more preferably 0.35 or less, mostpreferably 0.3 or less.

(4) Shape

To provide the honeycomb structure with both low pressure loss and highstrength, its cell walls preferably have thickness of 0.1-0.5 mm and acell pitch of 1-3 mm. The cross section shape of each flow path in thehoneycomb structure in a transverse direction is not particularlyrestricted, but it is preferably triangular, tetragonal, hexagonal orcircular from the aspect of production, particularly tetragonal from theaspect of strength and filter area. The cell walls preferably havelinearly extending flat surfaces, but may have curved surfaces as longas they do not give large resistance to an exhaust gas passing throughthe flow paths.

(5) Properties

The main crystals of aluminum titanate, in which MgO and SiO₂ aredissolved to form a solid solution, improve the thermal stability of thealuminum-titanate-based ceramic honeycomb structure of the presentinvention at 800-1250° C. Further, the glass phases containing MgO andSiO₂ introduce a large number of fine microcracks into the cell walls,providing the honeycomb structure with a small thermal expansioncoefficient, improved heat shock resistance and high strength.Specifically, the honeycomb structure can have A-axis compressionstrength of 4 MPa or more, a thermal expansion coefficient of 12×10⁻⁷/°C. or less, preferably 10×10⁻⁷/° C. or less. Accordingly, even whencommunicating pores are introduced into the cell walls to have lowpressure loss, thereby having as high porosity as 40-70%, the honeycombstructure can have practically sufficient strength.

With MgO and SiO₂ dissolved in aluminum titanate, as described above,the decomposition of aluminum titanate to rutile and corundum iseffectively prevented when the honeycomb structure is exposed to800-1250° C. for a long period of time, having 95% or more of aluminumtitanate remain when kept at 1100° C. in an atmosphere for 100 hours.

(6) Honeycomb Filter

As shown in FIGS. 1( a) and 1(b), a honeycomb filter 10 is obtained byalternately sealing the exhaust-gas-inlet-side end surface 8 andexhaust-gas-outlet-side end surface 9 of the outlet-side-sealed flowpaths 3 and the inlet-side-sealed flow paths 4 of the honeycombstructure in a checkerboard pattern. Plugs 6 a, 6 c for forming theoutlet-side-sealed flow paths 3 and the inlet-side-sealed flow paths 4are not necessarily disposed at end surfaces 8, 9 of the flow paths, butmay be disposed in portions of flow paths inside from the end surfaces.As shown in FIG. 4, more preferable is the formation of plugs 6 b insidethe inlet-side-sealed flow paths 4 on the exhaust gas inlet side fromthe aspect of the performance of cleaning an exhaust gas while capturingparticulate matter. The honeycomb structure shown in FIG. 4 can capturemore particulate matter in wider areas upstream of the inlet-side plugs6 b and the outlet-side plugs 6 c, than the conventional honeycombstructure shown in FIGS. 1( a) and 1(b) having plugs at both ends, andcan capture more particulate matter at the melt-down limit because maincrystals in the cell walls are made of heat-resistant aluminum titanate,in which MgO and SiO₂ are dissolved to form a solid solution, resultingin longer combustion intervals, and drastic improvement in the fuelefficiency of diesel engines. The outlet-side-sealed flow paths 3 andthe inlet-side-sealed flow paths 4 may have the same or differentopening areas.

[2] Production Method

The aluminum-titanate-based ceramic honeycomb structure of the presentinvention comprising at least main crystals of aluminum titanate, inwhich MgO and SiO₂ are dissolved to form a solid solution, and glassphases can be produced by mixing 100 parts by mass of a powder materialcomprising alumina powder containing 0.05-0.5% by mass of Na₂O andtitania powder containing 0.5% or less by mass of Na₂O at a molar ratioof 47/53 to 53/47, with at least 1-6 parts by mass of silica powder and0.5-5 parts by mass of magnesia source powder having an average particlesize of 5 μm or less, molding the resultant material to a honeycombstructure, and sintering the resultant molding by heating it at anaverage speed of 10° C./hr to 100° C./hr in a temperature range of 1300°C. to 1350° C. and keeping it at the highest keeping temperature of1400-1650° C.

(1) Starting Material Powder

The starting material powder comprises 100 parts by mass of a powdermaterial comprising alumina powder containing 0.05-0.5% by mass of Na₂Oand titania powder containing 0.5% or less by mass of Na₂O at a molarratio of 47/53 to 53/47, and at least 1-6 parts by mass of silica powderand 0.5-5 parts by mass of magnesia source powder having an averageparticle size of 5 μm or less.

With the alumina powder and the titania powder in a molar ratio range of47/53 to 53/47, the amounts of titania and alumina remaining after thesynthesis of aluminum titanate by sintering are reduced, so that thehoneycomb structure can have heat resistance inherent in aluminumtitanate. The molar ratio of titania powder to alumina powder ispreferably 48/52 to 52/48.

The alumina powder preferably has an average particle size of 0.2-10 μm.The average particle size of less than 0.2 μm may densifyaluminum-titanate-based ceramics with a smaller number of pores, needinga large amount of pore-forming materials to obtain porous bodies havingdesired porosity, average pore size, pore distribution deviation and airpermeability, and resulting in insufficiently low pressure loss. Whenthe average particle size exceeds 10 μm, the synthesis of aluminumtitanate becomes insufficient, so that alumina having a large thermalexpansion coefficient remains, failing to obtain a low thermal expansioncoefficient. The alumina powder more preferably has an average particlesize of 2-8 μm. Although small-particle-size alumina powder isconventionally used to produce aluminum titanate from alumina powder andtitania powder, the use of alumina powder having an average particlesize of 2-8 μm can suppress sintering shrinkage and permit pores toremain, thereby easily providing porous bodies with desired porosity,average pore size, pore distribution deviation and air permeability. Thealumina powder most preferably has an average particle size of 2-6 μm.The alumina powder preferably contains 50% or more by mass of powderhaving a particle size of 2-20 μm, and 5-30% by mass of powder having aparticle size of 20 μm or more. The average particle size of powder ismeasured by a laser diffraction method.

The titania powder preferably has an average particle size of 0.05-3 μm.When the average particle size is less than 0.05 μm, a large amount ofwater is needed to prepare a starting material, making cracking likelyin a drying step. When the average particle size is more than 3 μm, thesynthesis of aluminum titanate is insufficient. The titania powder morepreferably has an average particle size of 0.1-2 μm. The titania powderpreferably has purity of 98% or more. The crystal type of the titaniapowder may be anatase or rutile.

The alumina powder preferably has a larger average particle size thanthat of the titania powder. The use of large alumina powder and smalltitania powder provides a high-porosity structure with reduced sinteringshrinkage, thereby preventing sintering cracking even for as largehoneycomb structures as, for instance, 150 mm or more in outer diameterand 150 mm in length.

With 0.05-0.5% by mass of Na₂O contained in alumina powder and 0.5% orless by mass of Na₂O contained in titania powder, a low-melting-pointliquid phase can be generated mainly from silica powder and magnesiasource powder in a sintering process, and sintering bonds main crystalsof aluminum titanate with fine glass phases, resulting in finermicrocracks, a smaller thermal expansion coefficient and improvedstrength. Na₂O contained in the alumina powder is preferably 0.1-0.3% bymass, and Na₂O contained in the titania powder is preferably 0.01-0.3%by mass.

With 1-6 parts by mass of silica powder added per 100 parts by mass ofthe total of the titania powder and the alumina powder, the amount ofSiO₂ dissolved in main crystals of aluminum titanate can be 0.1-1.5%,and the area ratio of the glass phases to the total area of maincrystals of aluminum titanate and glass phases can be 2-12%, and/or themass ratio of glass phases per the total of main crystals of aluminumtitanate and glass phases can be 1% or more by mass and less than 10% bymass, both in a cross section of the sintered cell walls. The amount ofthe silica powder is more preferably 2-4 parts by mass.

The silica powder preferably has an average particle size of 0.1-30 μm.When the average particle size is more than 30 μm, the melting speed ofsilica is too slow to generate a liquid phase during sintering, makingfine microcracks less likely. In addition, when a plasticized moldablematerial passes through a die slit as narrow as, for instance, about 0.3mm during extrusion, the die slit is clogged with silica powder,resulting in a ceramic honeycomb structure with cracked cell walls. Whenthe average particle size is less than 0.1 μm, the silica powder has alarge specific surface area, needing much water in preparing a startingmaterial, and thus resulting in less self-supportable moldings andcracking in cell walls while drying. The silica powder more preferablyhas an average particle size of 1-25 μm. The silica may be quartz, andamorphous silica such as fused silica and colloidal silica, andamorphous silica obtained by completely fusing high-purity silica stonesat high temperatures is particularly preferable. As long as it isamorphous, amorphous silica may be pulverized powder or sphericalpowder. The purity of the silica powder is preferably 99% or more, morepreferably 99.5% or more. When silicon-containing minerals such asfeldspar are used, the amount of SiO₂ dissolved in main crystals ofaluminum titanate and the composition of glass phases cannot becontrolled within the range of the present invention. Accordingly,silica powder is used in the present invention.

With 0.5-5 parts by mass of magnesia source powder having an averageparticle size of 5 μm or less added to 100 parts by mass of the total oftitania powder and alumina powder, the amount of MgO dissolved in maincrystals of aluminum titanate can be 0.2-5%, and the amount of MgOcontained in glass phases can be 1-20%, resulting in a honeycombstructure with improved thermal stability at 800-1250° C., a smallthermal expansion coefficient and improved strength. The averageparticle size of more than 5 μm provides the magnesia source powder withreduced reactivity with the silica powder, resulting in less than 1% ofMgO contained in glass phases, and failing to provide the glass phaseswith a low melting point, and thus failing to provide a honeycombstructure with a small thermal expansion coefficient and high strength.The average particle size of the magnesia source powder is preferably0.2-4 μm, more preferably 0.2-2 μm. The amount of the magnesia sourcepowder is more preferably 0.5-2 parts by mass. The magnesia source maybe magnesium oxide, magnesium hydroxide, magnesium carbonate, talc,spinel, etc., and magnesium oxide is preferable from the aspect ofgenerating a liquid phase from alumina powder, titania powder and silicapowder in a sintering process.

(2) Sintering Conditions

In the sintering of a honeycomb structure molding, heating is conductedat an average speed of 10° C./hr to 100° C./hr in a temperature range of1300° C. to 1350° C. When ceramic products, particularly large honeycombstructures of 150 mm or more in outer diameter and 150 mm or more inlength, are sintered, the temperature-elevating speed is preferably aslow as possible to keep the temperature uniformity in a sinteringprocess. In the method of the present invention, however, thetemperature elevation speed is as high as 10° C./hr to 100° C./hr in atemperature range of 1300° C. to 1350° C., as a sintering condition forlarge honeycomb structures of 150 mm or more in outer diameter and 150mm or more in length, making microcracks finer. Though this reason isnot clear, it is considered that because in this temperature range,silica powder and magnesia source powder having an average particle sizeof 5 μm or less form a liquid phase, while alumina powder and titaniapowder synthesize aluminum titanate crystals, in which MgO and SiO₂ aredissolved to form a solid solution, main crystals of aluminum titanateare bonded with fine glass phases after sintering, resulting in finermicrocracks. The average temperature-elevating speed is more preferably30° C./hr to 80° C./hr, most preferably 50 to 80° C./hr. Although thetemperature may be elevated at 10° C./hr to 100° C./hr in a widertemperature range than 1300-1350° C., its range should be restricted to1250-1400° C. The temperature-elevating speed in other temperatureranges may be properly determined in a range not generating sinteringcracking in the honeycomb structure, but it should be lower than thetemperature-elevating speed in the above temperature range.

The sintering is conducted at 1400-1650° C. The synthesis of aluminumtitanate from alumina powder and titania powder is insufficient attemperatures lower than 1400° C., resulting in a large thermal expansioncoefficient. At temperatures higher than 1650° C., densificationprogresses when a pore-forming material described below is added,resulting in cell walls with low porosity and thus large pressure loss.It is also disadvantageous in a sintering cost. The more preferredsintering temperature is 1500-1600° C.

A time period in which the molding is kept at the highest temperature of1400-1650° C. during sintering is preferably 24 hours or longer. Thehighest-temperature-keeping time of 24 hours or longer makes fine poresdisappear, and the pore-forming material added acts to increase theaverage pore size, resulting in the honeycomb structure with large airpermeability and low pressure loss.

(3) Pore-Forming Materials

A starting material comprising alumina powder, titania powder, silicapowder and magnesia source powder is preferably mixed with apore-forming material. The addition of the pore-forming materialprovides the desired porosity, average pore size and air permeability.As the pore-forming material, flour, graphite, starch, cellulose,ceramic balloons, polyethylene, polystyrene, polypropylene, nylon,polyesters, acrylic resins, phenol resins, epoxy resins, ethylene-vinylacetate copolymers, styrene-butadiene block copolymers, styrene-isopreneblock copolymers, polymethyl methacrylate,methylmethacrylate-acrylonitrile copolymers, urethanes, waxes, etc. maybe used alone or in combination, and among them, resin foams made ofmethylmethacrylate-acrylonitrile copolymers are preferable.

The present invention will be explained in further detail by Examplesbelow without intention of restricting the present invention thereto.

Example 1

The rutile-type titania powder A and the alumina powder D shown in Table1 were weighed at a molar ratio of 50/50 (corresponding to a mass ratioof 43.9/56.1) (see Table 2), and 100 parts by mass of them were mixedwith 3 parts by mass of amorphous silica powder A (see Table 1), 1.0parts by mass of magnesia powder C (see Table 1), 14 parts by mass ofisobutane-containing resin foams, and 6 parts by mass of methylcellulose, and blended with water to form a plasticized moldablematerial. This plasticized moldable material was extruded to form amolding having a honeycomb structure. This molding was dried, and heatedin the air in a gas furnace for 150 hours from room temperature to 1000°C., at 10° C./h from 1000° C. to 1300° C., at 32° C./hr from 1300° C. to1350° C., and at a 10° C./h from 1350° C. to 1600° C., and kept at 1600°C. for 5 hours for sintering, thereby obtaining a honeycomb structure of152 mm in outer diameter, 152 mm in length, 0.25 mm in cell wallthickness and 1.4 mm in cell wall pitch, which had aluminum titanate asmain crystals.

Test pieces were cut out of this honeycomb structure to evaluate crystalphases constituting the honeycomb structure, the amounts of SiO₂ and MgOdissolved in main aluminum titanate crystals, the composition, averagesize, area ratio and mass ratio of glass phases, and the porosity,average pore size and pore distribution deviation σ of cell walls, airpermeability, A-axis compression strength, thermal expansion coefficient(CTE), and thermal stability at 1100° C.

The crystal phases constituting the honeycomb structure were identifiedby pulverizing the above test piece, measuring the X-ray diffractionpattern of the resultant powder sample by an X-ray diffractometer(RINT1500 available from Rigaku Corporation), and comparing it with thedata of the JCPDS file. The percentage of the crystal phases werecalculated from the ratios of the (230) plane diffraction intensity(I_(AT(230))) of aluminum titanate, the (104) plane diffractionintensity (I_(corundum(104))) of corundum Al₂O₃, the (110) planediffraction intensity (I_(rutile(110))) of rutile TiO₂, and the (311)plane diffraction intensity (I_(spinel(311))) of spinel.

The amounts of SiO₂ and MgO dissolved in main crystals of aluminumtitanate were determined by analyzing a ground cell wall cross sectionof the sintered honeycomb structure embedded in a resin by EPMA(EPMA-1610 available from Shimadzu Corporation), and comparing the X-raystrengths of Si and Mg in main crystals of aluminum titanate with thealready prepared calibration curve.

The composition, average size and area ratio of glass phases weredetermined by observing a ground cell wall cross section of the sinteredhoneycomb structure embedded in a resin by FE-SEM (S-4000 available fromHitachi, Ltd.). The composition of glass phases was measured by an EDXanalyzer, and the average size and area ratio (the ratio of the areas ofglass phases to the total area of aluminum titanate crystals and glassphases) of glass phases were calculated by image analysis software(Image-Pro Plus available from Media Cybernetics) on a SEM photographshowing the cross section of cell walls. The glass phases existedbetween crystal phases of aluminum titanate, and their maximum lengthswere regarded as their sizes. The average size of glass phases wasdetermined by measuring the sizes of 20 glass phases and averaging them.

The mass ratio (% by mass) a of glass phases was calculated by theformula of a=100×(d−b)/(c−b), wherein b represents the amount (% bymass) of SiO₂ dissolved in main aluminum titanate crystals, and crepresents the amount (% by mass) of SiO₂ in the glass phases, bothmeasured as described above, and d represents the amount (% by mass) ofSiO₂ in the starting material.

The porosity, average pore size and pore distribution deviationσ[=log(D20)−log(D80)] of cell walls of the honeycomb structure weremeasured by mercury porosimetry. A test piece (10 mm×10 mm×10 mm) cutout of the honeycomb structure was put in a measurement cell of AutoporeIII available from Micromeritics, and the measurement cell was evacuatedand pressurized by introducing mercury. From the relation of thepressure and the volume of mercury intruded into pores in the testpiece, the relation of a pore size and a cumulative pore volume wasdetermined. The mercury-introducing pressure was 0.5 psi (0.35×10⁻³kgf/mm²), and the pore size was calculated from the pressure using acontact angle of 130°, and a surface tension of 484 dyne/cm.

The air permeability of cell walls in the honeycomb structure wasmeasured by a known method on a disc-shaped test piece of 20 mm in outerdiameter cut out of the cell walls of the sintered honeycomb structure,using a perm porometer (CFP-1100 available from Porous Materials Inc.).

The A-axis compression strength was measured according to “Method OfTesting Monolith Ceramic Carriers For Cleaning Exhaust Gases FromAutomobiles,” Standard M505-87 of JASO.

The average thermal expansion coefficient (CTE) between 35° C. and 800°C. was determined on a test piece having a cross section of 4.8 mm×4.8mm and a length of 50 mm, which was cut out of the honeycomb structuresuch that its longitudinal direction was substantially aligned with thedirection of flow paths, by measuring increase in the longitudinallength of the test piece while heating it from 35° C. to 800° C. at atemperature-elevating speed of 10° C./minute under a constant load of 20g in a compression load/differential expansion-type thermomechanicalanalyzer (TMA, ThermoPlus available from Rigaku Corporation), andaveraging it.

The stability at 1100° C. was evaluated on a test piece of 10 mm×10mm×10 mm by measuring the ratio of aluminum titanate remaining after aheat treatment at 1100° C. for 100 hours in an air-atmosphere electricfurnace. The ratio (X) of the remaining aluminum titanate was determinedon the test piece before and after the above heat treatment, by theformula of X=[(R after heat treatment)/(R before heattreatment)]×100(%), wherein R is the ratio of aluminum titanaterepresented by [I_(AT(230))/(I_(TiO2(110))+I_(AT(230)))], whereinI_(AT(230)) is the X-ray diffraction intensity of a (230) plane ofaluminum titanate, and I_(TiO2(110)) is the X-ray diffraction intensityof a (110) plane of rutile TiO₂.

It was confirmed from X-ray diffraction measurement results that thehoneycomb structure of Example 1 contained aluminum titanate as maincrystals, and slightly corundum. It was further confirmed from EPMAanalysis and SEM observation that the honeycomb structure had glassphases containing MgO and SiO₂, together with main crystals of aluminumtitanate in which SiO₂ and MgO were dissolved. The amounts of SiO₂ andMgO dissolved in the main crystals, the composition, average size, arearatio and mass ratio of the glass phases, the porosity, average poresize and pore distribution deviation σ of cell walls, and the airpermeability, A-axis compression strength, thermal expansion coefficientand thermal stability at 1100° C. are shown in Table 3.

Examples 2-14 and Comparative Examples 1-6

The honeycomb structures of Examples 2-14 and Comparative Examples 1-6were produced in the same manner as in Example 1 except for changing thetypes, formulations and sintering conditions of powders used as shown inTable 2. Specifically, the following changes were made.

The honeycomb structure of Example 2 was produced in the same manner asin Example 1 except for using magnesium hydroxide as a magnesia sourcepowder, and changing the amount of resin foams added.

The honeycomb structure of Example 3 was produced in the same manner asin Example 1 except for changing the sintering temperature.

The honeycomb structure of Example 4 was produced in the same manner asin Example 1 except for changing the amounts of silica powder and resinfoams, and the temperature-elevating speed and the highest keepingtemperature during sintering.

The honeycomb structure of Example 5 was produced in the same manner asin Example 1, except for changing the mass ratio of rutile-type titaniapowder A to alumina powder D to 42.9/57.1 (49/51 by a molar ratio), thetype of silica powder, the amount of magnesia powder, and thetemperature-elevating speed during sintering.

The honeycomb structure of Example 6 was produced in the same manner asin Example 1 except for changing the amount of magnesia powder.

The honeycomb structure of Example 7 was produced in the same manner asin Example 1, except for changing the type and amount of magnesia powderand the sintering temperature.

The honeycomb structure of Example 8 was produced in the same manner asin Example 1 except for changing the amount of magnesia powder.

The honeycomb structure of Example 9 was produced in the same manner asin Example 1 except for changing the amount of magnesia and thesintering temperature.

The honeycomb structure of Example 10 was produced in the same manner asin Example 1, except for changing the type of alumina powder and thesintering temperature and time.

The honeycomb structure of Example 11 was produced in the same manner asin Example 1, except for changing the types of alumina powder and silicapowder, and the sintering temperature and time.

The honeycomb structure of Example 12 was produced in the same manner asin Example 1, except for using rutile-type titania powder A and aluminapowder C, changing their mass ratio to 44.9/55.1 (51/49 by a molarratio), the amount of silica powder, the type of magnesia powder, thetemperature-elevating speed during sintering, and the sinteringtemperature and time.

The honeycomb structure of Example 13 was produced in the same manner asin Example 12, except for changing the type of titania powder, theamount of silica powder, and the temperature-elevating speed duringsintering.

The honeycomb structure of Example 14 was produced in the same manner asin Example 13 except for changing the amount of silica powder and thetemperature-elevating speed during sintering.

The honeycomb structures of Comparative Examples 1 and 2 were producedin the same manner as in Example 1, except for using rutile-type titaniapowder A and alumina powder C, and changing their mass ratio to44.9/55.1 (51/49 by a molar ratio), the amount of silica powder, thetype of magnesia powder, and the sintering temperature and time.

The honeycomb structure of Comparative Example 3 was produced in thesame manner as in Example 1, except for changing the type of magnesiapowder, the temperature-elevating speed during sintering, and thesintering temperature and time.

The honeycomb structure of Comparative Example 4 was produced in thesame manner as in Example 1, except for using rutile-type titania powderA and alumina powder E, and changing their mass ratio to 41/59 (47/53 bya molar ratio), the type and amount of magnesia powder, the amount ofresin foams, and the sintering temperature and time.

The honeycomb structure of Comparative Example 5 was produced in thesame manner as in Comparative Example 4, except for changing the typesof titania powder and alumina powder, the type and amount of magnesiapowder, and the sintering temperature and time.

The honeycomb structure of Comparative Example 6 was produced in thesame manner as in Example 1 except for adding no magnesia powder.

With respect to the resultant honeycomb structures of Examples 2-14 andComparative Examples 1-6, crystal phases constituting them, the amountsof SiO₂ and MgO dissolved in main aluminum titanate crystals, thecomposition, average size, area ratio and mass ratio of glass phases,the porosity, average pore size and pore distribution deviation σ ofcell walls, and their air permeability, A-axis compression strength,thermal expansion coefficients (CTEs), and thermal stability at 1100° C.were evaluated in the same manner as in Example 1.

It was confirmed that the honeycomb structures of Examples 2-14 andComparative Examples 1-6 contained main crystals of aluminum titanate inwhich SiO₂ and MgO are dissolved to form a solid solution, and glassphases containing MgO and SiO₂ like that of Example 1.

TABLE 1 Average Ig. Particle Loss Chemical Composition (% by mass) ItemSize (μm) (%) Al₂O₃ TiO₂ SiO₂ MgO Na₂O K₂O Titania A 1.6 0.1 0.01 99.4 —0.11 0.022 0.08 (Rutile-Type) Rutile Titania B 0.8 0.1 0.01 99.5 — 0.10.031 0.07 (Rutile-Type) Alumina A⁽¹⁾ 0.5 0.1 99.9 — 0.04 — 0.03 —Alumina B⁽²⁾ 1 0.1 99.8 — 0.03 — 0.26 — Alumina C⁽³⁾ 3 0.1 99.8 — 0.02 —0.21 — Alumina D⁽⁴⁾ 5 0.05 99.7 — 0.02 — 0.25 — Alumina E⁽⁵⁾ 13 0.1 99.8— 0.02 — 0.32 — Silica A 20.1 0.1 0.08 — ≧99.5 — 0.0027  0.002(Amorphous) Silica B 1.8 0.1 0.13 — ≧99.5 — 0.0013 — (Amorphous) SilicaC 0.8 0.1 0.12 — ≧99.5 — 0.0012 — (Amorphous) Magnesia A 0.5 0.1 — — —99.9 0.02 — Magnesia B 0.8 0.1 — — — 99.8 0.01 — Magnesia C 1.3 0.1 — —— 99.9 0.01 — Magnesia D 2 0.1 — — — 99.98 0.014 — Magnesia E 5.1 0.1 —— — 99.8 0.01 — Magnesium 1.3 30.9 — — — 69 0.01 — Hydroxide Note:⁽¹⁾Alumina A contained 5% by mass of particles of 2-10 μm. ⁽²⁾Alumina Bcontained 12% by mass of particles of 2-10 μm. ⁽³⁾Alumina C contained54% by mass of particles of 2-10 μm, and 2% by mass of particles of 20μm or more. ⁽⁴⁾Alumina D contained 66% by mass of particles of 2-10 μm,and 7% by mass of particles of 20 μm or more. ⁽⁵⁾Alumina E contained 40%by mass of particles of 2-10 μm, and 20% by mass of particles of 20 μmor more.

TABLE 2 Formulation (parts by mass) Titania Alumina Silica No. A B A B CD E A B C Example 1 43.9 — — — — 56.1 — 3.0 — — Example 2 43.9 — — — —56.1 — 3.0 — — Example 3 43.9 — — — — 56.1 — 3.0 — — Example 4 43.9 — —— — 56.1 — — 3.0 — Example 5 42.9 — — — — 57.1 — — 3.0 — Example 6 43.9— — — — 56.1 — 3.0 — — Example 7 43.9 — — — — 56.1 — 3.0 — — Example 843.9 — — — — 56.1 — 3.0 — — Example 9 43.9 — — — — 56.1 — 3.0 — —Example 10 43.9 — — 56.1 — — — 3.0 — — Example 11 43.9 — — 56.1 — — — —— 3.0 Example 12 44.9 — — — 55.1 — — 1.0 — — Example 13 — 44.9 — — 55.1— — 4.0 — — Example 14 — 44.9 — 55.1 — — 5.0 — — Comparative 44.9 — — —55.1 — — 0.5 — — Example 1 Comparative 44.9 — — — 55.1 — — 8.0 — —Example 2 Comparative 43.9 — — — — 56.1 — 3.0 — — Example 3 Comparative41.0 — — — — — 59.0 3.0 — — Example 4 Comparative — 41.0 59.0 — — — —3.0 — — Example 5 Comparative 43.9 — — — — 56.1 — 3.0 — — Example 6Formulation (parts by mass) Magnesia Magnesium Resin No. A B C D EHydroxide Foams Example 1 — — 1.0 — — — 14 Example 2 — — — — — 1.44 12Example 3 — — 1.0 — — — 14 Example 4 — — 1.0 — — — 12 Example 5 — — 0.5— — — 14 Example 6 — — 3.0 — — — 14 Example 7 — — — 3.0 — — 14 Example 8— — 5.0 — — — 14 Example 9 — — 5.0 — — — 14 Example 10 — — 1.0 — — — 14Example 11 — — 1.0 — — — 14 Example 12 — 1.0 — — — — 14 Example 13 — 1.0— — — — 14 Example 14 — 1.0 — — — — 14 Comparative — 1.0 — — — — 14Example 1 Comparative — 1.0 — — — — 14 Example 2 Comparative — — — 1.0 —— 14 Example 3 Comparative — — — — 0.3 — 13 Example 4 Comparative 7.0 —— — — — 13 Example 5 Comparative — — — — — — 14 Example 6 SinteringConditions Temperature-Elevating Speed (° C./h) T_(h) ⁽¹⁾ Keeping No.1000-1300° C. 1300-1350° C. 1350° C. to T_(h) ⁽¹⁾ (° C.) Time (hr)Example 1 10 32 10 1600 5 Example 2 10 32 10 1600 5 Example 3 10 32 101500 5 Example 4 20 32 20 1500 5 Example 5 20 32 20 1600 5 Example 6 1032 10 1600 5 Example 7 10 32 10 1500 5 Example 8 10 32 10 1600 5 Example9 10 32 10 1500 5 Example 10 10 32 10 1460 10 Example 11 10 32 10 143010 Example 12 10 20 10 1550 24 Example 13 20 50 20 1550 24 Example 14 2060 20 1550 24 Comparative 10 32 10 1550 10 Example 1 Comparative 10 3210 1550 10 Example 2 Comparative 10 10 10 1660 10 Example 3 Comparative10 32 10 1450 10 Example 4 Comparative 10 32 10 1650 10 Example 5Comparative 10 32 10 1600 5 Example 6 Note: ⁽¹⁾T_(h) means the highesttemperature.

TABLE 3 Crystal Phases (%) Amount (% by mass) Other Than Main CrystalsIn Main Crystal Phases No. Corundum Spinel Rutile SiO₂ MgO Example 1 2 00 0.50 0.80 Example 2 1 0 0 0.49 0.78 Example 3 3 0 0 0.28 0.81 Example4 2 0 0 0.30 0.78 Example 5 1 0 0 0.51 0.45 Example 6 8 0 0 0.33 1.84Example 7 9 1 0 0.35 2.57 Example 8 0 12 0 0.33 2.47 Example 9 0 11 00.36 2.67 Example 10 2 0 0 0.50 0.78 Example 11 1 0 0 0.48 0.81 Example12 2 0 0 0.20 0.80 Example 13 1 0 0 0.63 0.76 Example 14 2 0 0 0.75 0.78Comparative 1 0 0 0.10 0.77 Example 1 Comparative 0 0 0 0.94 0.76Example 2 Comparative 1 7 0 0.60 0.76 Example 3 Comparative 2 0 0 0.600.10 Example 4 Comparative 0 14 0 0.80 0.82 Example 5 Comparative 1 0 00.50 0 Example 6 Composition (% by mass) Of Glass Phases No. SiO₂ MgOAl₂O₃ TiO₂ Na₂O K₂O Example 1 63.4 7.4 23.1 1.6 3.2 1.2 Example 2 63.27.5 23.2 1.6 3.3 1.2 Example 3 62.1 3.9 23.8 5.7 2.7 1.8 Example 4 61.64.2 23.9 5.8 2.8 1.7 Example 5 66.3 3.5 22.8 3.9 2.5 1.0 Example 6 62.68.0 22.4 2.8 2.4 1.8 Example 7 60.2 8.0 25.3 2.1 3.0 1.4 Example 8 66.95.5 22.0 2.2 2.4 1.0 Example 9 70.8 3.6 18.9 2.9 2.3 1.4 Example 10 63.77.3 22.9 1.8 3.1 1.1 Example 11 62.1 5.5 23.8 5.2 2.5 0.9 Example 1249.6 6.2 34.5 5.8 2.8 1.1 Example 13 69.1 3.6 21.2 3.5 1.8 0.7 Example14 69.9 3.5 20.7 4.2 1.2 0.5 Comparative 44.5 8.8 36.4 6.6 2.4 1.3Example 1 Comparative 70.3 4.2 19.7 3.3 1.7 0.8 Example 2 Comparative63.2 6.8 23.4 3.1 2.4 1.1 Example 3 Comparative 71.2 1.0 18.0 7.2 2.00.6 Example 4 Comparative 63.1 7.1 25.2 3.3 0.8 0.5 Example 5Comparative 75.1 0 11.4 9.4 1.5 2.6 Example 6 Glass Phases CrystalPhases Average Size Area Ratio Mass Ratio Number Of No. (μm) (%) (% bymass) Microcracks Example 1 14.5 3.8 3.5 89 Example 2 15.1 4.0 3.6 110Example 3 2.5 4.2 4.2 124 Example 4 3.2 3.9 4.1 137 Example 5 14.5 4.84.2 96 Example 6 12.0 4.5 4.0 89 Example 7 3.2 4.2 3.9 117 Example 813.1 4.9 3.7 103 Example 9 2.5 4.6 3.4 106 Example 10 5.6 5.2 4.0 82Example 11 6.3 4.9 3.9 96 Example 12 9.8 2.0 1.6 82 Example 13 5.2 7.54.9 96 Example 14 6.3 8.1 6.1 69 Comparative 6.5 0.5 0.9 48 Example 1Comparative 11.4 13.0 10.1 34 Example 2 Comparative 32.0 3.8 3.8 48Example 3 Comparative 25.0 4.0 3.4 48 Example 4 Comparative 31.5 5.8 4.269 Example 5 Comparative 23.5 3.5 3.6 34 Example 6 Cell Walls AveragePore Pore Air Porosity Size Distribution Permeability No. (%) (μm)Deviation σ (×10⁻¹² m²) Example 1 45.2 23.5 0.24 2.4 Example 2 42.3 24.50.23 2.5 Example 3 55.3 18.2 0.21 2.1 Example 4 50.0 12.0 0.26 1.8Example 5 52.3 21.5 0.27 2.2 Example 6 46.0 27.5 0.21 3.2 Example 7 52.018.0 0.24 1.8 Example 8 47.5 22.0 0.22 2.4 Example 9 57.8 19.8 0.25 2.2Example 10 51.0 13.8 0.25 1.6 Example 11 49.0 10.5 0.23 1.2 Example 1258.0 20.0 0.22 2.8 Example 13 56.0 21.2 0.32 2.7 Example 14 57.0 19.90.35 2.4 Comparative 54.0 18.7 0.25 2.2 Example 1 Comparative 42.2 15.70.25 1.8 Example 2 Comparative 44.5 24.0 0.24 2.6 Example 3 Comparative58.1 22.3 0.22 3.0 Example 4 Comparative 42.3 12.9 0.32 0.9 Example 5Comparative 57.2 18.0 0.25 2.8 Example 6 A-Axis Thermal ExpansionThermal Compression Coefficient Stability No. Strength (MPa) (×10⁻⁷/°C.) at 1100° C.⁽¹⁾ Example 1 6.0 1.7 98 Example 2 6.5 −4.2 99 Example 34.5 4.8 98 Example 4 5.8 1.2 97 Example 5 4.1 3.8 98 Example 6 6.2 3.699 Example 7 4.5 6.3 99 Example 8 5.7 4.6 98 Example 9 4.2 5.2 99Example 10 7.0 2.3 98 Example 11 8.0 9.7 98 Example 12 4.0 11.2 96Example 13 5.5 7.8 98 Example 14 3.8 11.5 99 Comparative 1.5 14.0 93Example 1 Comparative 4.0 14.0 97 Example 2 Comparative 4.5 13.0 98Example 3 Comparative 1.8 15.0 82 Example 4 Comparative 7.0 15.0 99Example 5 Comparative 2.5 22.0 51 Example 6 Note: ⁽¹⁾Expressed by theamount (%) of aluminum titanate remaining after kept at 1100° C. in anatmosphere for 100 hours.

As shown in Tables 1-3, it was confirmed that the honeycomb structuresof Examples 1-14 within the present invention produced with molar ratiosof titania powder to alumina powder in a range of 47/53 to 53/47, andwith 1-6 parts by mass of silica powder and 0.5-5 parts by mass ofmagnesia powder having an average particle size of 5 μm or less addedbased on the total amount of titania powder and alumina powder, attemperature-elevating speeds of 10° C./hr or more between 1300° C. and1350° C. and sintering temperatures of 1450-1650° C., contained maincrystals of aluminum titanate in which SiO₂ and MgO are dissolved toform a solid solution, and glass phases containing MgO and SiO₂, theglass phases having area ratios of 2-8% and containing 2.5-15% of MgO.These honeycomb structures had porosities of 40-70%, average pore sizesof 10-40 μm, pore distribution deviations of 0.4 or less, airpermeability of 1.2×10⁻¹² m² or more in cell walls, A-axis compressionstrengths of 3.8 MPa or more, and thermal expansion coefficients of12×10⁻⁷/° C. or less in cell walls, the ratios of the aluminum titanatecrystals remaining when kept at 1100° C. for 100 hours being 95% ormore, thereby having low pressure loss, high strength, high heat shockresistance, and high thermal stability at 1100° C.

On the other hand, the honeycomb structure of Comparative Example 1, inwhich the amount of silica was less than 1%, had a small area ratio ofglass phases, resulting in low A-axis compression strength and a largethermal expansion coefficient. The honeycomb structure of ComparativeExample 2, in which the amount of silica was more than 6%, had a largearea ratio of glass phases, resulting in a low A-axis compressionstrength and a large thermal expansion coefficient. The honeycombstructure of Comparative Example 3 produced by a sintering temperaturehigher than 1650° C. had a large average size of glass phases, resultingin a low A-axis compression strength and a large thermal expansioncoefficient. The honeycomb structure of Comparative Example 4, in whichmagnesia had an average particle size of more than 2 μm, the amount ofmagnesia was less than 0.5%, and alumina had an average particle size ofmore than 10 μm had glass phases containing less than 2.5% of MgO, a lowA-axis compression strength, and a large thermal expansion coefficient.The honeycomb structure of Comparative Example 5, in which the amount ofmagnesia was more than 5%, had glass phases containing more than 15% ofMgO, with cell walls having small air permeability and a large thermalexpansion coefficient. The honeycomb structure of Comparative Example 6,to which magnesia was not added, had a small number of microcracks,resulting in a low A-axis compression strength and a large thermalexpansion coefficient. Further, because MgO was not dissolved in maincrystals of aluminum titanate, it had extremely low thermal stability.

Example 15

The honeycomb structure produced in Example 1 had a shape having anouter diameter of 152 mm, a length of 152 mm, a cell wall thickness of0.25 mm and a cell wall pitch of 1.4 mm, and porosity of 45.2%, anaverage pore size of 23.5 μm, a pore distribution deviation of 0.24 andair permeability of 2.4×10⁻¹² m². This honeycomb structure had maincrystals of aluminum titanate, in which MgO and SiO₂ are dissolved toform a solid solution, and glass phases. After its flow paths werealternately plugged by a known method in a checkerboard pattern at bothends, a peripheral portion of this honeycomb structure was removed bymachining to an outer diameter of 141 mm. Longitudinal grooves definedby peripheral cell walls were filled with a coating material containing5 parts by mass, on a solid basis, of colloidal silica per 100 parts bymass of aluminum titanate powder, and dried to form an outer peripheralwall, thereby providing a honeycomb filter having the structure shown inFIGS. 1( a) and 1(b), which had an outer diameter of 144 mm, a length of152 mm, a cell wall thickness of 0.25 mm, and a cell wall pitch of 1.4mm.

Comparative Example 7

Using cordierite-forming materials, a practical cordierite-basedhoneycomb filter having the structure shown in FIGS. 1( a) and 1(b) wasproduced by a known method. This honeycomb filter had an outer diameterof 152 mm, a length of 152 mm, a cell wall thickness of 0.3 mm, a cellwall pitch of 1.57 mm, porosity of 63%, an average pore size of 22 μm, apore distribution deviation of 0.7, and air permeability of 3.2×10⁻¹²m².

Ceramic honeycomb filters obtained in Example 15 and Comparative Example7 were measured with respect to pressure loss characteristics. Thepressure loss characteristics were evaluated by pressure differencebetween upstream and downstream of the ceramic honeycomb filter fixed toa pressure loss test stand, when air was supplied at a flow rate of 4Nm³/min. As a result, the honeycomb filter of Example 15 had pressureloss of 100 mmAq, while the honeycomb filter of Comparative Example 7had pressure loss of 90 mmAq. It was confirmed that the honeycomb filterof Example 15 produced by using the aluminum-titanate-based ceramichoneycomb structure of the present invention had low pressure losscharacteristics equal to those of the practical cordierite-basedhoneycomb filter.

EFFECT OF THE INVENTION

Optimizing the compositions and sizes of main crystals of aluminumtitanate, in which MgO and SiO₂ are dissolved to form a solid solution,and glass phases existing therebetween, the aluminum-titanate-basedceramic honeycomb structure of the present invention have improved heatshock resistance, pressure loss characteristics, thermal stability(800-1250° C.) and strength, particularly suitable for large ceramichoneycomb filters for diesel engines, which have high porosity and outerdiameters of more than 200 mm.

The method of the present invention can optimize the composition andsize of glass phases in aluminum-titanate-based ceramics comprising maincrystals of aluminum titanate, in which MgO and SiO₂ are dissolved toform a solid solution, and glass phases, thereby easily producing thealuminum-titanate-based ceramic honeycomb structure of the presentinvention.

Using the starting material powder of the present invention, thealuminum-titanate-based ceramic honeycomb structure of the presentinvention can be produced easily.

What is claimed is:
 1. A ceramic honeycomb structure having a largenumber of flow paths partitioned by porous cell walls, said cell wallscomprising at least main crystals of aluminum titanate, in which MgO andSiO₂ are dissolved to form a solid solution, and glass phases; saidglass phases containing 40-80% by mass of SiO₂ and 1-20% by mass of MgO;the average size of said glass phases being 30 μm or less in a crosssection of said cell walls; and the area ratio of said glass phases tothe total area of said main crystals of aluminum titanate and said glassphases being 2-7.5% in a cross section of said cell walls.
 2. Thealuminum-titanate-based ceramic honeycomb structure according to claim1, wherein the amounts of MgO and SiO₂ dissolved in said main crystalsare 0.2-5% by mass and 0.1-1.5% by mass, respectively.
 3. Thealuminum-titanate-based ceramic honeycomb structure according to claim1, wherein said porous cell walls have porosity of 40-70%, an averagepore size of 10-40 μm, and a pore distribution deviationσ[=log(D20)−log(D80)] of 0.4 or less, wherein D20 represents a pore size(μm) at a pore volume corresponding to 20% of the total pore volume, andD80 represents a pore size (μm) at a pore volume corresponding to 80% ofthe total pore volume, both in a curve representing the relation betweenthe pore size and the cumulative pore volume (pore volume accumulated ina range from the maximum pore size to a particular pore size), andD80<D20.
 4. The aluminum-titanate-based ceramic honeycomb structureaccording to claim 1, wherein said cell walls have air permeability of2×10⁻¹² m² or more.
 5. The aluminum-titanate-based ceramic honeycombstructure according to claim 1, wherein said honeycomb structure has anA-axis compression strength of 4 MPa or more.
 6. Thealuminum-titanate-based ceramic honeycomb structure according to claim1, wherein said cell walls have a thermal expansion coefficient of12×10⁻⁷/° C. or less.
 7. The aluminum-titanate-based ceramic honeycombstructure according to claim 1, wherein 95% or more of the aluminumtitanate remains after said cell walls are kept in an atmosphere at1100° C. for 100 hours.
 8. A ceramic honeycomb structure having a largenumber of flow paths partitioned by porous cell walls, said cell wallscomprising at least main crystals of aluminum titanate, in which MgO andSiO₂ are dissolved to form a solid solution, and glass phases; saidglass phases containing 40-80% by mass of SiO₂ and 1-20% by mass of MgO;the average size of said glass phases being 30 μm or less in a crosssection of said cell walls; and the mass ratio of said glass phases tothe total of said main crystals of aluminum titanate and said glassphases being 1.6-4.9% by mass.
 9. A starting material powder forproducing an aluminum-titanate-based ceramic honeycomb structurecomprising at least main crystals of aluminum titanate, in which MgO andSiO₂ are dissolved to form a solid solution, and glass phases, saidstarting material powder comprising 100 parts by mass of a powdermaterial comprising alumina powder containing 0.05-0.5% by mass of Na₂Oand titania powder containing 0.01-0.5% by mass of Na₂O at a molar ratioof 47/53 to 53/47, and at least 1-6 parts by mass of silica powder, and0.5-5 parts by mass of magnesia source powder having an average particlesize of 5 μm or less.
 10. The starting material powder for producing analuminum-titanate-based ceramic honeycomb structure according to claim9, wherein said alumina powder contains 0.21-0.5% by mass of Na₂O.